1. Field of the Invention
The present invention relates generally to a process for converting Fischer-Tropsch olefin-containing light fractions into high octane alkylates. More particularly, this invention relates to a process of using a catalytic dehydration/isomerization reaction to convert Fischer-Tropsch light olefin fractions into feeds for alkylation.
2. Description of Related Art
Because of their high octane numbers and low vapor pressures, alkylates have been used for many years as blending components in motor gasolines. The alkylation process involves the reaction of light olefins such as propylene and butylene with isoparaffins such as isobutane and isopentane in the presence of an acid catalyst such as H2SO4 and HF to form highly branched, isoparaffinic products known as alkylates. Typical sources for light olefins include catalytic crackers, cokers and vis-breakers.
The Fischer-Tropsch process also produces light olefins. The process involves reacting synthesis gas composed mainly of CO and H2 in the presence of a suitable catalyst to form a variety of predominantly linear hydrocarbonaceous solid, liquid and gaseous products. Some of these products can be refined using known procedures such as hydrotreating, hydrocracking and hydroisomerization to yield moderately branched, isoparaffin-rich middle distillate fuels such as diesel and jet fuels. A gaseous phase produced in a Fischer-Tropsch synthesis, upon condensation and subsequent distillation, produces a light olefin product composed primarily of C3-C4 olefins. However, this product is considered to be poorly suited for use as a feedstock for conversion to alkylates for a number of reasons.
Products of Fischer-Tropsch syntheses normally contain relatively high levels of oxygenates, frequently in amounts above 4000 ppm oxygen. Oxygenates can react to form water which would dilute acid catalysts conventionally used in alkylation. Accordingly, light olefin feedstocks for alkylation should contain no more than 4000 ppm oxygenates, preferably much less. Also, light olefin streams from Fischer-Tropsch processes have relatively low levels of isobutane. As such, they do not constitute a good source for isobutane. In the alkylation process, isobutane feedstocks should contain at least 30% by weight, preferably up to 75% by weight isobutane. Also, Fischer-Tropsch light olefin streams are composed predominantly of 1-butene. It has been observed that 1-butene tends to give alkylates with lower octane numbers than 2-butene. Desirably, the ratio of 2-butene to total butenes in the feed for alkylation should be at least 0.1 and most preferably at least 0.5.
Hydroisomerization processes have been used to produce moderately branched isoparaffins in the distillate fuel boiling range. However, they cannot be used to produce high-octane, highly branched isoparaffins in the gasoline boiling range. If hydroisomerization processes are run in a severe mode in an attempt to create highly branched products instead of moderately branched products, the feedstock cracks to form excessive amounts of undesirable light gases. Accordingly, the only practical method to manufacture high octane, highly branched, gasoline boiling range isoparaffins is by alkylation.
Consideration has been given to converting Fischer-Tropsch C3-C4 olefin streams into a feedstock suitable for alkylation by total hydrogenation or by using known treatments to lower the oxygenate content to an acceptable level. However, these approaches would necessitate a separate step to isomerize butane while hydrogenation would saturate the olefins in the C4 stream. Also, the oxygenates removed from the C3-C4 stream would not be converted into alkylates. What is needed is an economical process for converting a C3-C4 light olefin fraction from a Fischer-Tropsch process into a highly branched isoparaffin alkylate suitable as a blending component to prepare high octane gasolines.
Alkylation using light olefins obtained via a Fischer-Tropsch synthesis are described in U.S. Pat. Nos. 4,279,830; 4,046,830; and 4,049,741. These patents do not disclose using a combined dehydration/isomerization process to reduce oxygenate levels and increase the amount of olefins and 2-butene.
It is an object of the invention to provide a process for economically preparing high octane alkylates from a Fischer-Tropsch C3-C4 light olefin stream.
It is another object of the invention to utilize a Fischer-Tropsch C3-C4 olefin fraction and an isobutane stream obtained by hydrocracking a Fischer-Tropsch 300xc2x0 F.+ product to produce a high octane alkylate.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description, the claims appended hereto and the FIGURE of the drawing.
The objectives of the invention are attained by a process which includes the following steps:
(a) recovering a light C3-C4 olefin product stream from a Fischer-Tropsch reactor;
(b) contacting the olefin stream with a dehydrating/isomerizing catalyst to dehydrate oxygenates in the olefin stream to form additional olefins and also convert 1-butenes to 2-butenes;
(c) hydrocracking a C5+ product from a Fischer-Tropsch synthesis to generate an isobutane-containing stream;
(d) blending the product stream from step (b) which contains less than about 4000 ppm oxygenate, with the product stream from step (c);
(e) alkylating the blend of step (d); and
(f) recovering a highly branched, isoparaffinic alkylate having a research octane number greater than 80.
A key feature of the invention lies in the discovery that certain dehydration catalysts can serve a dual function. On the one hand, they dehydrate linear alcohols present in the C3-C4 olefin stream (e.g., 1-propanol and 1-butanol) to provide additional C3 and C4 olefins (which also reduces the amount of oxygenates present in the feedstock), and on the other hand, they isomerize 1-butenes to 2-butenes which can increase the octane value. Since the C3-C4 olefin feed used in the alkylation stage should contain no more than about 4000 ppm oxygenate and preferably less than 1000 ppm, it is desirable to treat the C3-C4 olefin stream to remove at least a portion of organic acids present and at least a portion of the alcohols which have not been dehydrated. Suitable known techniques to accomplish this include extractive or azeotropic distillation, decarboxylation, adsorption and water extraction or water washing.